I. Field of the Invention
The present invention relates generally to a steerable system for controlling borehole deviation with respect to the vertical axis by varying the angle of such deviation without removing (tripping) the system from the borehole, and more particularly to a directional drilling apparatus that is remotely adjustable or variable during operation for affecting deviation control.
II. Description of the Prior Art
The technology developed with respect to drilling boreholes in the earth has long encompassed the use of various techniques and tools to control the deviation of boreholes during the drilling operation. One such system is shown in U.S. Pat. No. Re. 33,751, and is commonly referred to as a steerable system. By definition, a steerable system is one that controls borehole deviation without being required to be withdrawn from the borehole during the drilling operation.
The typical steerable system today comprises a downhole motor having a bent housing, a fixed diameter near bit stabilizer on the lower end of the motor housing, a second fixed diameter stabilizer above the motor housing and an MWD (measurement-while-drilling) system above that. A lead collar of about three to ten feet is sometimes run between the motor and the second stabilizer. Such a system is typically capable of building, dropping or turning about three to eight degrees per 100 feet when sliding, i.e. just the motor output shaft is rotating the drill bit while the drill string remains rotationally stationary. When rotating, i.e. both the motor and the drill string are rotating to drive the bit, the goal is usually for the system to simply hold angle (zero build rate), but variations in hole conditions, operating parameters, wear on the assembly, etc. usually cause a slight build or drop. This variation from the planned path may be as much as .+-.one degree per 100 feet. When this occurs, two options are available. The first option is to make periodic corrections by sliding the system part of the time. The second option is to trip the assembly and change the lead collar length or, less frequently, the diameter of the second stabilizer to fine tune the rotating mode build rate.
One potential problem with the first option is that when sliding, sharp angle changes referred to as doglegs and ledges may be produced, which increase torque and drag on the drill string, thereby reducing drilling efficiencies and capabilities. Moreover, the rate of penetration for the system is lower during the sliding mode. The problem with the second option is the costly time it takes to trip. In addition, the conditions which prevented the assembly from holding angle may change again, thus requiring additional sliding or another trip.
The drawbacks to the steerable system make it desirable to be able to make less drastic directional changes and to accomplish this while rotating. Such corrections can readily be made by providing a stabilizer in the assembly that is capable of adjusting its diameter or the position of its blades during operation.
One such adjustable stabilizer known as the Andergage, is commercially available and is described in U.S. Pat. No. 4,848,490. This stabilizer adjusts a half-inch diametrically, and when run above a steerable motor, is capable of inclination corrections on the order of.+-.one-half a degree per 100 feet, when rotating. This tool is activated by applying weight to the assembly and is locked into position by the flow of the drilling fluid. This means of communication and actuation essentially limits the number of positions to two, i.e. extended and retracted. This tool has an additional operational disadvantage in that it must be reset each time a connection is made during drilling.
To verify that actuation has occurred, a 200 psi pressure drop is created when the stabilizer is extended. One problem with this is that it robs the bit of hydraulic horsepower. Another problem is that downhole conditions may make it difficult to detect the 200 psi increase. Still another problem is that if a third position were required, an additional pressure drop would necessarily be imposed to monitor the third position. This would either severely starve the bit or add significantly to the surface pressure requirements.
Another limitation of the Andergage is that its one-half inch range of adjustment may be insufficient to compensate for the cumulative variations in drilling conditions mentioned above. As a result, it may be necessary to continue to operate in the sliding mode.
The Andergage is currently being run as a near-bit stabilizer in rotary-only applications, and as a second stabilizer (above the bent motor housing) in a steerable system. However, the operational disadvantages mentioned above have prevented its widespread use.
Another adjustable or variable stabilizer, the Varistab, has seen very limited commercial use. This stabilizer is covered by the following U.S. Pat. Nos. 4,821,817; 4,844,178; 4,848,488; 4,951,760; 5,065,825; and 5,070,950. This stabilizer may have more than two positions, but the construction of the tool dictates that it must index through these positions in order. The gauge of the stabilizer remains in a given position, regardless of flow status, until an actuation cycle drives the blades of the stabilizer to the next position. The blades are driven outwardly by a ramped mandrel, and no external force in any direction can force the blade to retract. This is an operational disadvantage. If the stabilizer were stuck in a tight hole and were in the middle position, it would be difficult to advance it through the largest extended position to return to the smallest. Moreover, no amount of pipe movement would assist in driving the blades back.
To actuate the blade mechanism, flow must be increased beyond a given threshold. This means that in the remainder of the time, the drilling flow rate must be below the threshold. Since bit hydraulic horsepower is a third power function of flow rate, this communication-actuation method severely reduces the hydraulic horsepower available to the bit.
The source of power for indexing the blades is the increased internal pressure drop which occurs when the flow threshold is exceeded. It is this actuation method that dictates that the blades remain in position even after flow is reduced. The use of an internal pressure drop to hold blades in position (as opposed to driving them there and leaving them locked in position) would require a constant pressure restriction, which would even be more undesirable.
A pressure spike, detectable at the surface, is generated when activated, but this is only an indication that activation has occurred. The pressure spike does not uniquely identify the position which has been reached. The driller, therefore, is required to keep track of pressure spikes in order to determine the position of the stabilizer blades. However, complications arise because conditions such as motor stalling, jets plugging, and cuttings building up in the annulus, all can create pressure spikes which may give false indications. To date, the Varistab has had minimal commercial success due to its operational limitations.
With respect to the tool disclosed in U.S. Pat. No. 5,065,825, the construction taught in this patent would allow communication and activation at lower flow rate thresholds. However, there is no procedure to permit the unique identification of the blade position. Also, measurement of threshold flow rates through the use of a differential pressure transducer can be inaccurate due to partial blockage or due to variations in drilling fluid density.
Another adjustable stabilizer recently commercialized is shown in U.S. Pat. No. 4,572,305. It has four straight blades that extend radially three or four positions and is set by weight and locked into position by flow. The amount of weight on bit before flow initiates will dictate blade position. The problem with this configuration is that in directional wells, it can be very difficult to determine true weight-on-bit and it would be hard to get this tool to go to the right position with setting increments of only a few thousand pounds per position.
Other patents pertaining to adjustable stabilizers or downhole tool control systems are listed as follows: U.S. Pat. Nos. 3,051,255; 3,123,162; 3,370,657; 3,974,886; 4,270,619; 4,407,377; 4,491,187; 4,572,305; 4,655,289; 4,683,956; 4,763,258; 4,807,708; 4,848,490; 4,854,403; and 4,947,944.
The failure of adjustable stabilizers to have a greater impact on directional drilling can generally be attributed to either lack of ruggedness, lack of sufficient change in diameter, inability to positively identify actual diameter, or setting procedures which interfere with the normal drilling process.
The above methods accomplish control of the inclination of a well being drilled. Other inventions may control the azimuth (i.e. direction in the horizontal plane) of a well. Examples of patents relating to azimuth control include the following: U.S. Pat. Nos. 3,092,188; 3,593,810; 4,394,881; 4,635,736; and 5,038,872.